1. Field of the Invention
The present invention relates to a liquid crystal display device (LCD), and more particularly to a backlight of the liquid crystal display device.
2. Discussion of the Related Art
A liquid crystal display device is widely used for a note book computer or a portable TV and generally comprises a liquid crystal panel and a backlight to provide a uniform collimated light to the liquid crystal panel. The liquid crystal panel basically comprises an upper substrate, a lower substrate, and a liquid crystal layer between the two substrates.
As shown in FIG. 1, the backlight generally comprises a light source 1; a light source housing 3 both for protecting the light source 1 and for concentrating light from the light source 1; a light pipe or light guide 4; a reflecting plate 6 on a lower surface of the light pipe 4; a light dispersing film 7 over an upper surface of the light pipe 4; one or two lens films 8 over the light dispersing film 7; and a protecting film 9 over the lens films 8. The backlight provides a uniform collimated light to the liquid crystal panel.
Scattering patterns are formed on a predetermined region of the light pipe 4 to scatter light which is trapped within the light pipe 4 by total internal reflection. The light pipe 4 is divided into a non-display region A and a display region B.
Light emitted from the light source 1 is incident on a light entering surface 2 of the light pipe 4, and being totally reflected, the incident light travels within the light pipe 4 by the non-display and display regions A and B in the named order. The light trapped within the light pipe 4 is scattered by the scattering patterns, destroying the total internal reflection condition, and the light partially comes out of the light pipe 4 through the upper surface so that a uniform collimated light is provided to the liquid crystal panel. The light reflecting plate 6 under the light pipe 4 reflects light coming out of the light pipe 4 through its lower surface, improving the light utilization ratio. The scattering film 7 imparts an optimum uniformity and directionality to the exit light. The lens film 8 concentrates the light into a predetermined direction, preventing a bad image. The protecting film 9 protects the lens film 8, increases the light uniformity, and enlarges the viewing angle range by properly dispersing the light.
As mentioned above, after entering the light pipe 4 through the light entering surface 2, the light exits from the light pipe 4 when scattered by the scattering patterns. The density of the scattering patterns is determined so that the display region B is provided with a uniform collimated light.
However, as shown in FIGS. 2a or 2b, light can enter the upper and lower surfaces of the light pipe 4 or the rounded surfaces 2a and 2b through a gap between the light source housing 3 and the light pipe 4. Most of the light which enters the light pipe 4 through the upper surface of the light pipe 4 is directed to the display region B without total internal reflection and light dispersion, resulting in a bright line. Referring to light paths shown in the Figures, it can be understood that the position of the bright line on the upper surface of the light pipe 4 is changed according to its incident angle, and is changed according to the exit angle.
FIG. 2c is a graph showing the exit position (Y, mm) with respect to the exit angle (X, degree) of the bright line mentioned above. where the tested value T is compared with the computed value C. As shown in FIG. 2d, the exit position Y is defined as a horizontal distance between the incident surface 2 and the exit position, and the exit angle X is defined as an angle between the normal direction and the exit direction S. This test and computation are performed with a light pipe having a thickness of 2.6 mm and without any optical film such as a dispersing film, and a lens film.
Two causes of the bright line can be seen by FIG. 2a and FIG. 2b, respectively. The relationship shown in FIG. 2C applies to both cases. When there is a large gap between the light source housing 3 and the light pipe 4, the bright line of FIG. 2a increases, and when there are large rounded surfaces 2a and 2b on the light pipe 4, the bright line of FIG. 2b increases. The rounded surfaces 2a and 2b are results from an inaccurate molding process. It is more difficult to prevent the bright line caused by the rounded surfaces in comparison, with the bright line of the gap between the light source housing 3 and the light pipe 4. Thus, the light pipe 4 is typically manufactured to make the edges without the rounded surfaces. Accordingly, the following explanations are mainly given for the bright line caused by the gap between the light pipe and the light source housing.
As shown in FIG. 3, printed regions 11, 12, and 13 are conventionally formed, respectively, on the light pipe 4, reflecting plate 6, and the dispersing plate 7 in the light path of the bright line in order to prevent the bright line. In this figure, the same elements as in FIG. 1 are indicated by same numerals without further explanation of the structure. Light incident on the upper surface of the light pipe 4 is weakened by absorption and scattering by the black printed region 11 on the upper surface of the light pipe 4, and the weakened light is further weakened by absorption, reflection and scattering by the black printed region 12 on the reflecting plate 6. Finally, the weakened light is scattered and removed by the white printed region 13 on the dispersing film 7.
Light entering a gap between the lower surface of the light pipe 4 and the reflecting plate 6 also is weakened by absorption and scattering by the black printed region 12, and then scattered and removed by the white printed region 13 on the dispersing film 7.
The structure described above to prevent the bright line has the following problems.
First, the manufacturing process is complicated to form the printed regions on the light pipe 4, the reflecting plate 6, and the dispersing film 7, and print degradation is easily generated, increasing the manufacturing cost.
Second, the printing regions are liable to weaken light which does not cause the bright line, decreasing the brightness of the collimated light provided to the liquid crystal panel. When using all of the printing regions on the light pipe 4, reflecting plate 6, and the dispersing film 7 to entirely remove the bright line, the light brightness is lowered by 3%-6%.
Third, a black stripe image comes in sight when using a deep black printing region, namely, it is difficult to determine a proper depth of black.
Fourth, there is a possible error of 0.5 mm in the vertical positions of the reflecting plate 6 and the dispersion film 7. Thus, the reflecting plate 6 may enter the light source housing 3, so that the black printing region 12 absorbs light incident on the light entering surface 2, decreasing the light brightness of the backlight. Further, the reflecting plate 6 may be so far removed from the light source housing 3 as to fail to remove the bright line by the black printing region 12. Further, if the dispersion film 7 is not correctly located, white and black stripes can be seen vaguely. Actually, the vague white and black strips are inevitable unless the reflecting plate 6 and the dispersing film 7 are fixed.
Accordingly, the present invention is directed to a backlight of a liquid crystal display device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a backlight of a liquid crystal display device which can provide a bright and uniform collimated light without generating a bright line.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the present invention provides a backlight of a liquid crystal display device which comprises:
a light pipe having a first surface as a top surface, a second surface as a bottom surface, and an end surface as a light entering surface, wherein the first surface is connected to the end surface through a first boundary and the second surface is connected to the end surface through a second boundary;
a light source facing the light entering surface; and
a light source housing including a light concentrating portion and a fixing portion; wherein the light concentrating portion has an inner space where the light source is located and an opening facing the end surface; wherein the opening is defined by first and second sides and at least one of the two sides is located vertically between the first boundary and the second boundary; and wherein the fixing portion is connected to the light concentrating portion through the opening and has an end located on at least one of the first and second surfaces.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.